Rotary engines have promise of high efficiencies, high power densities and low part count, which have attracted numerous engineers and efforts to this filed. Among great many configurations existing in the prior art, one of the simplest and the most promising is based on the gerotor concept. With reference to FIGS. 1(a)-1(d), depicting prior art, and more specifically to FIG. 1(a), a gerotor includes an outer rotor rotatably mounted within the housing cavity (housing not shown) and having the female gear profile and inner rotor with male gear profile. In a course of its operation, both outer and inner rotor rotate within housing, forming a plurality of successive chambers of variable volume. These chambers could be used to execute compression or expansion of gas in compressors/pneumatic motors/engines or movement of liquids in pumps/hydraulic motors. An alternative configuration is for an outer rotor to be stationary while inner rotor wobbles, driven by an eccentric shaft. The variable volume chambers thus formed behave similarly to the first configuration. Frictional losses associated with these designs could be reduced by using a roller-vane gerotor design shown in FIG. 1(b). Instead of direct contact between the outer and inner rotors, rollers are incorporated to form the displacement chambers. In all of these designs an outer rotor is used not only to form the chambers but also to guide the inner rotor.
Having very few moving parts it is not surprising that this simple design has attracted attention of many who attempted to design a rotary engine around it. The major problem that could be traced to all rotary engines, however, is difficulty in sealing the working fluid during the compression, combustion and expansion strokes of the engines. While theoretically most of the engines look feasible on paper, since they completely encompass working fluid without using seals, in practice, when machining tolerances and thermal expansion are taken into account and also when parts are starting to wear out, the sealing of working fluid is not possible without seals. The most famous version of gerotor-based engine and the only one used in production is the Wankel engine, in which 3-lobe rotor moves inside of 2-lobe housing, as shown in FIG. 1(c). This engine was relatively successful for two main reasons. First, the outer rotor was not used to guide the inner rotor, but rather a pair of gears was used to synchronize the movement and rotation of the inner rotor with motion of the eccentric shaft. Second, the gap between the inner rotor and outer rotor, which is provided to allow for manufacturing tolerances, thermal expansion, and wear, was sealed by a grid work of seals, may be known as a “Wankel Grid”, consisting of face seals located on flat part of the rotor and apex seals located within each apex of the rotor, and also “buttons” that connected both of these types of seals; all of these seals are located on the rotor, and therefore will move with the rotor. Together with rotor itself and the housing, in theory these seals completely encompassed the working fluid. Again, in practice, there are still gaps between the seals or seals and rotor and seals and housing, but these are relatively small and manageable and enable the engine to function. Having said this, it is well known that these engines have relatively low efficiency and high emissions and are unsuitable for compression ignition mode of operation due to:
1. Relatively high degree of leakage, despite the seal grid. For example, the bouncing of fast-moving apex seals, as well as holes into the engine to accommodate spark plug(s), contribute to leakage.
2. Large seal travel.
3. High thermal losses caused by very high surface to volume ratio of the combustion chamber at the moment of highest compression.
4. Low geometrically achievable compression ratio.
5. Necessity to meter oil into the working chamber to lubricate the apex seals, which can't get lubrication by any other means, and the existence of ports through which this oil is exhausted, causing emission problems.
Theoretically gerotor engines with a stationary outer rotor have just one major moving part—the rotor. This rotor, moving inside a housing, forms variable geometry cavities that contract and expand in a course of rotor's rotation. The sealing is accomplished by theoretical line contact between the rotor and the housing; such a contact is to occur at least in two places. In general, the gerotors are designed to have very small sliding contact between the rotor and the housing, though, attempts were made to implement “rolling without sliding”—see U.S. Pat. No. 7,520,738 to Katz as an example of such an effort. Another example is described in U.S. Pat. No. 5,373,819 to Rene Linder, which uses rollers in conjunction with an eccentric to guide a rotor within the housing. Yet another example is described in Russian patent RU 2078221 C1 to Veselovsky, which uses seals within a housing. In practice, as stated above, manufacturing tolerances and thermal expansion cause designers to leave a relatively large gap between the rotor and the housing or rotor and rollers. If housing and rotor are inflexible or if rollers can't accommodate for the thermal expansion or the preload due to machining tolerance, the sealing can't be accomplished. So, it becomes meaningless to talk about purely rolling contact between rotor and the housing. This gap has to be closed one way or another by the seal to enable a workable engine.